1. Field of the Invention:
This invention relates generally to a clutch pressure modulator valve device and more specifically to a clutch pressure modulator valve for modulating the clutch pressure applied to a fluid pressure operated clutch of power transmission of a particular motorized vehicle, for example a fork lift.
2. Description of the Prior Art:
In a conventional clutch pressure modulator valve device, the valve device comprises a piston disposed in an accumulator chamber for accumulating oil pressure operating a clutch of a power transmission and a relief port for transmitting a part of the pressurized oil in the accumulator chamber to an oil reservoir upon movement of the piston, whereby the oil pressure operating clutch in the accumulator chamber increases along a gradual pressure gradient to a predetermined value. Subsequently, communication between the accumulator chamber and the relief port is cut off by movement of the piston after the oil pressure in the accumulator chamber applied to operating clutch is increased to the predetermined value. In this type of valve, however, the effective diameter of the relief port for determining the pressure gradient of the modulating characteristic of the oil pressure is fixed as to its general value and cannot be controlled from the exterior of the power transmission. Although it is desirable that the modulating characteristic of the pressure gradient of the modulator valve changes in response to an operating condition or load condition of the vehicle in order to operate the clutch under an optimum pressure gradient by, for example, changing a spring force biasing the piston, the volume of accumulator chamber or the flow rate of oil into the accumulator chamber. As a result, this can cause a shock to be generated due to the sudden engagement of the clutch when the vehicle is under a light load due to too steep a pressure gradient modulating characteristic or can cause an increase in time lag of clutch operation when the vehicle is under a heavy load due to too gradual a pressure gradient modulating characteristic.